In a variety of navigation applications, it is necessary to determine the heading of an accelerating object. The heading of an object is the direction the object is pointing. For example, a robotic airplane may require heading information and other vehicle state information to point a camera at a target, navigate to a destination, and otherwise control its movement. Conventional systems measure heading using a traditional floating magnetic compass or strap-down magnetic vector sensor to measure the Earth's magnetic field. However, these sensors respond to any magnetic field. Thus, these sensors are subject to local magnetic disturbance. Many techniques exist for calibrating out local magnetic disturbance that is non-time variant with respect to the magnetic sensor reference frame. However, arbitrary time-varying magnetic disturbance is common and can cause significant heading measurement error. A typical example of a time-varying disturbance is a vehicle driving past a stationary magnetic sensor on a tripod.
True heading is heading determined with respect to the Earth's spin axis. True heading is used in navigation applications to geo-reference a map and account for accelerating inertial frame navigation corrections. To determine true north from a magnetic sensor requires a magnetic model of the Earth as a function of position and time. Such models are limited in accuracy because they do not account for local geographic mass distribution and cannot precisely predict the change to Earth's magnetic field in the future.
Alternatively, Global Navigation Satellite System (GNSS) such as the Global Positioning System (GPS), Galileo or GLONASS are used to calculate position and velocity of an object. Based on the position and velocity, a velocity track in an Earth-fixed reference frame can be derived. The velocity track represents the direction an object is moving, which is then assumed to be the direction an object is pointing. For precise navigation of ground or air vehicles, this assumption is incorrect. Therefore, the heading determined based on the GNSS systems tend to be inaccurate. Further, GNSS systems are also subject to jamming and poor reception of satellite signals, reducing the availability of a GNSS heading determination.
A gyro-compassing system is also used to determine true heading. The gyro-compassing system uses the Earth's rotational rate vector as a reference to directly determine true heading. Traditional gyro-compassing and north-finding systems are based on mechanical, Ring-Laser Gyro (RLG) or Fiber Optic Gyro (FOG) technologies. Because the Earth's rotational rate vector is unaffected by interference, gyro-compassing is highly reliable and self-contained. However, these systems are relatively large, heavy and expensive for man-portable or small robotic vehicle applications. Thus, deployment of the gyro-compassing system is generally limited to military vehicles, commercial aircrafts, and spacecrafts.